OK, you want foolproof? Videos/tv and a snack to munch. Sorry, that's what I've been doing for naps about 85% of the time, ever since ds2 needed quiet for getting to sleep.
But - there are other substitutes that also sometimes work for ds1. He'll just as happily, and for longer stretches, play computer games. Noggin, Curious George games, Sesame Street games, etc. Sometimes I give him the choice of computer or tv and he chooses computer. But I do explain to him (he knows by now though) that if something goes wrong on the computer and he can't find his way back to his games, he still needs to do something quiet until I come down. And I do let him turn on the tv.
Sometimes a new favorite CD and access to the controls - and he's happy with that until I come down. More so recently, the last couple months, he can engage in a book for the whole time, but that's pretty rare.
I have been known to reward him with a treat (cookie or animal crackers, or chocolate milk, etc.) if he was quiet and waited until I came downstairs. One time recently I came down to find that the power in the house went out while he was watching TV. He got a book and just waited for me, no yelling or anything. That boy earned himself some ice cream that day!
I don't do the rewards much anymore. He's matured a bit over the past year, and has gotten used to the routine by now - ds2 is 2yo now, after all!